This invention relates generally to gas turbine engines and more particularly to apparatus for cooling turbine nozzles in such engines.
A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine (“HPT”) in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine includes annular arrays (“rows”) of stationary vanes or nozzles that direct the gases exiting the combustor into rotating blades or buckets. Collectively one row of nozzles and one row of blades make up a “stage”. Typically two or more stages are used in serial flow relationship. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life.
HPT nozzles are often configured as an array of airfoil-shaped vanes extending between annular inner and outer bands which define the primary flowpath through the nozzle. Some prior art HPT nozzles have experienced temperatures on the aft inner band above the design intent. This has lead to the loss of the aft inner band because of oxidation at a low number of engine cycles. The material loss can trigger a chain of undesirable events, leading to serious engine failures. For example, in a multi-stage HPT, the loss of the aft portion of the first stage nozzle inner band can cause hot gas ingestion between the first stage nozzle and the forward rotating seal member or “angel wing” of the adjacent first stage blade. The ingested primary flow can in turn heat up the forward cooling plate of the first stage rotor disk causing it to crack. Once the cooling plate is cracked, hot air can heat up the first stage rotor disk causing damage to the disk post, which could lead to the release of a first stage turbine blade.
The inner bands of prior art HPT nozzles often have a pocket of material removed therefrom, for the purposes of weight reduction. However, in the presence of high velocity flow, as seen under a typical inner band, this pocket can cause a stagnation region. The stagnation region degrades cooling and can lead to the failures described above.